Variable frequency magnetron circuit



June 7, 1949- I E. EVERHART 2,472,200

VARIABLE FREQUENCY MAGNETRON CIRCUIT I Filed Aug. 8, 1945 L!) (9 n LEAKAGE REGION CURRENT FIG. 3

SOURCE OF FLUX REACTNCE MAGNETRON MAGNETRON OSCILLATOR AF m MEGACYLES I5 IO 4o 0-0 WATTS DISSIPATED m o 7 I00 200300400500 I REACTANCE TUBE REACTANCE MAGNETRON INVENTOR 0-0 ANODE VOLTAGE EDGAR EVERHART aw aw 9% ATTORNEY Patented June 7, 1949 UNITED STATES PATENT OFFICE VARIABLE FREQUENCY MAGNETRON CIRCUIT Edgar Ever-hart, Camb mesne assignments,

"must employ thermionic tubes which operate relatively efilciently at those frequencies. As herein used, the term hyper frequencies will irefer to those frequencies at which conventional coils, condensers andresistors may no longer be used to practical advantage. Some such device as a cavity resonator mustbe used, for the resonant circuit at such frequencies. The magnetron is such a tube which can be made to operate relatively efliciently at these hyper frequencies. The multi-cavity magnetron is particularly applicable for use as a reactance tube because of a change in dielectric constant with change in cathode-to-anode voltage when the tube is run below cut-off, at too low voltage to cause conduction with the existing magnetic field.

and the magnetron operated below cut-ofi acts.

as a rapidly tunable cavity. When tightly coupled into a transmitter oscillator, the reactance tube 'achieves such rapid electronic tuning as is desired for frequency modulation purposes. The frequency modulation system, and especially its reactance magnetron tuner, is lossless with respect to RF. energy; that is, the RF. output of the transmitter does not vary measurably with tuning. There is an amount of D. C. power absorbed in the reactance tube due to what is knownas end-space leakage, which will. be described later, but such loss is not too great and it is not the loss of the useful R.F. energy.

The multi-cavity magnetron is one in which the anode comprises a number of cavity resonators placed symmetrically about the cathode each ridge, Mass., assignor, by I to "the United States of America as represented by the Secretary of War Application August a. 1945, Serial No. 609,651

9 Claims. (Cl. 332-) cavity being coupled to the cathode-anode space by means of slots.

Accordingly it is an object of this invention to provide a magnetron oscillator coupled to the other magnetron of similar physical dimensions as the dimensions of the oscillator magnetron, thesecond magnetron acting as a variable re. actance tube'for controlling the frequency of the magnetron oscillator.

Still another object of this invention is to pro 'vide a magnetron oscillator whose frequency is modulated by a reactance magnetron coupled to the magnetron oscillator, the reactance of the reactance magnetron being varied by varying its cathode-anode voltage.

- Still another object of this invention is to provide a magnetron oscillator whose frequency may be either modulated or adjusted by a reactance magnetron coupled to it, the reactance magnetron having a cathode-to-anode diameter ratio in the order of .60-.85 and a flux density in the order of 230 Accordingly, among the objects of the present invention are: a

1. To provide a magnetron oscillator for use at hyper frequencies; and

2. To provide a method of using such a magnetron as a reactance tube.-

. In accordance with the present invention there is provided a multi-cavity magnetron oscillator with a normal high anode voltage and large magnetic field, loosely coupled to its load. Tightly coupled to this magnetron oscillator is a reactance magnetron, having a normal large magnetic field, but a very small anode voltage, below the cut-ofl'. region of voltage as hereinafter referred to. As the anode voltage of the reactance magnetron is varied, its output reactance is varied and the frequency of oscillation of the magnetron oscillater is changed.

. This invention will best be understood by reference to the drawings, in which:

' Fig. 1 is the voltage-current characteristic for a multi-cavity magnetron;

Fig. 2 is a simplified drawing ofa multi-cavity magnetron showing the structure of the fields existing in the cathode-anode space;

. Fig. 3 is a functional diagramof a magnetron oscillator controlled by a reactance magnetron;

Fig. 4 is a graph of variation in magnetron oscillator frequency ,(Ajl as a function of the D.-C. anode voltage of the reactance magnetron; and

. Fig. 5 is a graph of the variation in D.-C. power dissipated in the reactance magnetronas a limetion of frequency change in the magnetron oscillator.

Referring now to a description of the operation of the apparatus and to Fig. i, it is seen that as the anode voltage of a multi-cavity magnetron is increased from zero, the anode current increases slowly at first and then suddenly starts a rapid increase. The knee of this curve is the so-called cut-ofi region of the magnetron oscillator. At lower voltages the tube current is due to "leakage current, not current that reaches the anode by conventional multi-cavity magnetron action. At voltages higher than those at the knee of the curve, the magnetron oscillates in a normal manner.

This characteristic may be explained by noting the fields existing in the cathode-anode space. shown highly simplified in Fig. 2, in which a cutaway view of the magnetron is given. A portion of anode vanes III, II, It, and is, which are the walls separating the various cavity resonators, are shown with alternate instantaneous positive and negative electric charges upon them, which is the situation for one desirable mode of operation. The anode electric field vectors M, ii, l8. l1 and II may then take the curved paths substantially as shown. The cathode is of the magnetron is shown situated symmetrically with respect to the anode vanes. At low anode voltages, the space charge 20 which surrounds the cathode, shown functionally in Fig. 2, extends only part way across the cathode-anode space. The magnetic field must be large in order for this condition to occur.

An increase in anode voltage causes an extension of the space charge toward the anode, with the result that the effective dielectric constant of the cathode-anode space of the magnetron is changed. The capacitive reactance and thus the resonant frequency of the magnetron changes with variation in D.-C. anode voltage, and thus the cut-oi! magnetron acts like a rapidly tunable cavity. It is desirable that the reactance magnetron II, Fig. 3, and the magnetron oscillator 3| be reasonably tightly coupled together for the reactance magnetron to have the optimum effect. One possible arrangement involves the use of a line. connecting the two magnetrons, having an effective length of a multiple of a half wavelength n)./2. at the operating frequency, nL/ 2, where n is any integer and A is the operating wavelength of the system. With this arrangement, rapid electronic tuning may be accomplished for frequency modulation or automatic frequency control p poses.

A typical graph of the amount of D.-C. anode volts required on the reactance magnetron to effect a given change in frequency of the magnetron oscillator is shown in Fig. 4. This curve is for continuous-wave oscillations from one magnetron oscillator of approximately watts power level. The change in radio-frequency power over the electronic tuning range was found to be not more than 10 per cent in this case.

There is an amount of D.-C. power absorbed in the reactance magnetron due in part to end-space leakage, which involves the flow of electrons around the end plates and out to the magnetron case; The amount of D.-C. power required in a typical case, the constants, being the same as those used to obtain the graph of Fig. 4, is shown in Fig. 5.

It was found that there is a relationship between the range of electronic tuning obtainable and the cathode-to-anode diameter ratio. 0f

4 three typical magnetrons tried, the first with per cent cathode-to-anode diameter ratiogave a tuning range of 3 megaeycles': the second with 75 per cent cathode-to-anode diameter ratio gave 9 megacycles range: and the third having an per cent ratio gave a range of 20 megacycles, the a tered. The timing is substantially a maximum and the R.-F. loss is substantially a minimum when the magnetic field B is in the region of 2130. At B=Bc there was a maximum of loss. and at B= 3Bo the reactance magnetron cathode became too hot and no tuning advantage was found.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications'as fall within the true spirit and scope of the invention.

What is claimed is:

1. A reactance controlled oscillator comprising a first multicavity magnetron supplied with a cathode-anode voltage and a magnetic field of conventional values for causing said first magnetron to operate as an oscillator in one of its resonant modes, a second multicavity magnetron having a system of resonant modes, one of the modes of said second magnetron having substantially the same resonant wave length as the wave length of one of the modes of said first magnetron said second magnetron being supplied with a magnetic field and an adjustable cathode-anode voltage to cause said second magnetron to act as a reactance whose value is determined by the cathode-anode voltage, and a coupling circuit interconnecting said magnetrons, whereby said second magnetron acts as an adjustable reactance capable of modifying the reactance of the resonance cavities of said first magnetron for adjusting the frequency of said first magnetron.

2. In combination, a first magnetron having a system of resonant modes. said magnetron being provided: with a magnetic field and a cathodeanode voltage producing oscillations corresponding to one of said modes, a second multicavity magnetron having a system of resonant modes. one of the modes of said second magnetron having substantially the same resonant wave length as the wave length of one of the modes of said first magnetron, with the intensity of the magnetic field of said second magnetron being on the order of 2(10,700/)\) where A is the operating wave length of said first magnetron. and the cathodeanode voltage of said second magnetron being below the "cut-off" voltage of said second magnetron, and means for closely coupling said first and second magnetrons whereby said second magnetron acts as a reactance capable of infiuencing the frequency of oscillation of said first magnetron.

3. A reactance controlled oscillator comprising, a first oscillating magnetron having a system of resonant modes, a second magnetron, means for intercoupling said first and second magnetrons, means for providing a magnetic field and a cathode-anode voltage for'said first magnetron of the magnitude setting said first magnetron into oscillations corresponding to one of said modes, and means for providing a cathode-anode" voltage for the second magnetron below its "cutoil voltage, the cathode-to-anode diameter ra-' tio of said second magnetron being on the order of .85, and a magneticfield to make said second magnetron to act as a reactance, influencing the frequency of oscillation of said first magnetron.

4. A reactance controlled oscillator comprising, a first oscillating magnetron having'a system of resonant modes, a second magnetron, means for intercoupling said first and second magnetrons, means for providing a magnetic field and a cathode-anode voltage for said first magnetron of the magnitude setting said first magnetron'into oscillations corresponding to one of said modes, and

system of resonant modes, one of the modes of said second magnetron having substantially the same resonant wave length as the wave length of one of the modes of said first magnetron, said second magnetron being supplied with a mag- ]netic field and a varying cathode-anode voltage,

the 'm'aximum value of'sald varying cathodeanode voltage being below the cut-oil" voltage of said second magnetron to cause said second magnetron to act as a reactance varying in response to the variation in the cathode-anode voltage of said second magnetron, and a coupling circuit interconnecting said .magnetrons whereby said second magnetron frequency modulates the output of said first magnetron.

- 7. In combination, a first oscillating magnetron, a second magnetron tightly coupled to said first magnetron, each of said magnetrons having a cathode and an anode, said anodes having a number of cavity resonators, means to impress a variable voltage onto the cathode-anode circuit means for providing a cathode-anode voltage,

and a magnetic fleld to make said second magnetron to act as a reactance influencing the frequency of oscillation of said first mag-netron the density of the magnetic flux of said second magnetron being on the order of 1 where A is the operating wave length of said first magnetron. I

5. A rea'ctance controlled oscillator comprising, a firstoscillating magnetron having a system of resonant modes, a second magnetron, means for intercoupling said first and second magnetrons comprising a concentric line having an effective length of where n is any integer and A is the operating wave length of the first magnetron, means for ,providinga magnetic field and a cathode-anode voltage for said first magnetron of the magnitude setting said first magnetron into oscillations corresponding to one of said modes, and means for.

providing a cathode-anode voltage for the second magnetron below its "cut-oi!" voltage, and a 1 magnetic field to make said second magnetron to act as a reactance influencing the frequency of oscillation of said first magnetron 6. A frequency modulated system comprising a first multicavity magnetron having a 'system' of said second magnetron, said voltage beingbe: low the cut-o region of said second magnetron to cause avariation in the resonant frequency of said first magnetron.

8. In combination, a first multicavity oscillating magnetron, a second multicavity magnetron tightly coupled to said flrst'magnetron, a load ofl region of the corresponding magnetron to I file of this patent:

of resonant modes, said first magnetron being supplied with a cathode-anode voltage and a magnetic field of conventional values for causing said first magnetron to operate as an oscillator, a second multicavity magnetron" having a 1 produce oscillations, the other said voltage being variable below the cut-ofl region of the other magnetron to vary the resonant frequency of said pairor magnetrons.

EDGAR EYERHART;

( REFERENCES crrnn' The following references are of record in the sums ra'mn'rs I Number Name Date 2,241,976 Blewett May 13, 1941 2,404,212 Bondley July 16, 1948- 2,408,286 Spencer Sept. 24, 1946 2,412,373 Usselman- Dec. 10, 19 

